Internal Process Controls - Control of Color Imaging Systems: Analysis and Design - page 476

9.3 TIME HIERARCHICAL PROCESS CONTROL LOOPS
The controls required for color printers are signi

cantly more complex as compared
with those required for black and white printers, not only due to the use of multiple
primaries, but also due to high print quality requirements for color due to the eye s

'

sensitivity to small color variations. Given the traditional dif culties associated with the

control of color, a multilevel modular control architecture was proposed [2,3] that is
conceptually very similar to other hierarchical control systems [4]. Time hierarchy
comes from the

rule used to design complex control systems,
which transforms the printing system to many simpler subsystems while preserving the
overall performance goals. Each controller sees the controllers below it as a virtual body
from which it gets information and sends commands. In a control hierarchy, the lower
level controllers run faster than the higher level loops, controlling a group of subsystem
variables at a higher rate. The lower level controllers deliver simpler view to higher level
controls. The higher level controls coordinate commands to subsystems at a much lower
rate. In Xerox, levels 1, 2, 3, and 4 controls are used to describe the time hierarchy of
process controls. Level 1 includes the lower level subsystem controls such as the
''

reduction of complexity

'' ''

etc., level 2 the controls between
subsystems (e.g., '' charge and development '' systems), level 3 the image control for
each separation tone adjustments (e.g., 1-D tone reproduction control), and level 4 the
image control between multiple separation tone adjustments (e.g., 2-D LUT and 3-D
pro

charge control, toner concentration control,

'' '' ''

les) to minimize the interactions between colorants that cause color shift in the
output. The control functions managing the job scheduling and managing set points
based on media attributes (on a sheet-by-sheet basis) are done by constraint-based
schedulers that are executed at a different level. Control functions that require

for example, redirecting jobs to an available printer
when another fails, are several layers higher than the subsystem level controls.
Each layer in the architecture is characterized by the nature of its sensor input,
actuator output, and algorithm properties. An abstraction of the architecture for levels 1,
2, and 3 is presented for process control loops schematically in Figure 9.2a for an image-
on-image (IOI) printing systemwith a belt photoconductor. Similar abstraction is applic-
able to a typical printing system that is non-IOI, as in the drum photoconductor system.

human = operator-in-the loop,

'' ''

9.4 LEVEL 1 ELECTROSTATIC CONTROL SYSTEM
The level 1 controller is closely coupled to its related subsystem. It operates at the
subsystem level to control subsystem parameters directly. Both its sensing and
actuations occur locally. At this level, the actuations and the sensed parameters are
coupled by a single process step. An important feature of the level 1 controller is that
there is a simple and direct relationship between the sensed and controlled para-
meters. The algorithms of these lower level controls must be both noise immune and
able to respond rapidly to changes in set points since their set points will be altered
by the higher level algorithms.
It is important to understand the principal subsystems involved in the design of
the level 1 controller. The xerographic process is centered on the photoconductor,
which is a multilayer belt or drum that retains charge in the dark but discharges when